Polymer electrolyte fuel cells have advantages that the power generation efficiency is high, the power density is high, rapid start and stop are possible, and downsizing and weight reduction are possible, and the applications of the polymer electrolyte fuel cells to portable power sources, power sources for movement, small-sized power generators for stationary use, and so on are expected.
In the polymer electrolyte fuel cells, platinum or platinum alloy is generally used as a catalyst in order to facilitate oxygen reduction reaction that occurs on the positive electrode, but resources of platinum are extremely small and platinum is expensive, which is a large obstacle to put platinum into practical use. Thus, a carbon catalyst exhibiting oxygen reduction activity by containing nitrogen and/or a transition metal is getting a lot of attention as an electrode catalyst for fuel cells, the catalyst not needing a noble metal such as platinum.
As an experiment for improving the oxygen reduction activity, in Patent Literature 1, for example, a carbon catalyst formed by assembling, in a non-aggregated state, a carbonized body of shell-like structure having an average particle diameter of 10 to 20 nm is synthesized. Moreover, in Patent Literature 2, a carbon catalyst having an average particle diameter of 317 to 451 nm is synthesized by carbonizing a polymer whose particle shape is controlled.